This is the U.S. application that claims priority to the German Application 101 10 392.1, filed Mar. 3, 2001.
1. Field of the Invention
The present invention relates to an improved process for preparing phenols.
2. Description of the Related Art
The process of acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone has been of particular industrial importance for a long time. In the preparation of phenol from cumene by the Hock process, cumene is oxidized to cumene hydroperoxide (CHP) in a first reaction step, known as oxidation, and the CHP is subsequently concentrated to from 65 to 90% by weight in a vacuum distillation, known as concentration. In a second reaction step, known as cleavage, the CHP is cleaved into phenol and acetone by action of an acid, usually sulfuric acid. In this step, the dimethyl phenyl carbinol (DMPC) formed in the oxidation is partly cleaved in an equilibrium reaction into xcex1-methylstyrene (AMS) and water, while a further part of the DMPC reacts with CHP to form dicumyl peroxide (DCP); the rest remains in the cleavage product. After neutralization of the cleavage product, this product mixture is usually worked up by distillation.
In the cleavage, part of the AMS or of the DMPC forms high boilers (dimers, cumylphenols, bisphenols) which are discharged as residue in the distillation. The AMS still present after the neutralization, is hydrogenated to cumene in the distillation and is returned to the oxidation. DMPC which is not reacted in the cleavage ends up as high boiler in the residue; part of it reacts further in the hot phenol columns to form AMS from which high-boiling secondary components are once again formed. DCP is stable at customary cleavage temperatures (50xc2x0 C.-70xc2x0 C.). It decomposes thermally in the hot phenol columns forming, in our experience, o-cresol, at least in part. On the other hand, in the presence of acid, DCP can be cleaved into phenol, acetone and AMS at temperatures above 80xc2x0 C. It is therefore obvious for the remaining DMPC and the DCP formed in the cleavage to be reacted completely immediately after the cleavage by means of a targeted increase in the temperature in the presence of the acid used as catalyst in the cleavage. In this way, DMPC is largely converted into AMS and DCP is converted virtually completely into phenol, acetone and likewise AMS.
Such a thermal after-treatment of the cleavage product has already been described in U.S. Pat. No. 2,757,209, where temperatures above 100xc2x0 C., specifically from 110xc2x0 C. to 120xc2x0 C., were employed. The objective of this thermal after-treatment was the complete dehydration of DMPC to AMS. On the other hand, U.S. Pat. No. 4,358,618 describes a thermal after-treatment which has the aim of converting all of the DCP formed in the cleavage into phenol, acetone and AMS; in that patent, temperatures of 120xc2x0 C. and 150xc2x0 C. are employed. U.S. Pat. No. 5,254,751 describes a thermal after-treatment which has the same objective as that in U.S. Pat. No. 4,358,618 and uses temperatures of from 80xc2x0 C. to 110xc2x0 C. Finally, in DE 197 55 026 A1, the after-treatment is carried out in a temperature range above 150xc2x0 C. In all these processes known from the prior art, the thermally treated product is subsequently cooled to (customarily) 40xc2x0 C. by means of a cooler, then neutralized and, after separating off a salt-containing aqueous phase, worked up by distillation.
A disadvantage of the above-described processes is that hydroxyacetone and other carbonyl compounds such as acetaldehyde are formed as by-products and these, firstly, make the work-up of the reaction product difficult and, secondly, hydroxyacetone in particular reacts with phenol in specific phenol purification processes to form high boilers, thus leading to undesirable losses of phenol. It would therefore be desirable to reduce the content of hydroxyacetone and other impurities in the cleavage product.
U.S. Pat. No. 6,066,767 describes a process for removing hydroxyacetone and other carbonyl compounds from the product of the cleavage of cumene hydroperoxide. For this purpose, the reaction product of the cumene hydroperoxide cleavage is extracted with an aqueous salt solution in a temperature range of 15xc2x0-80xc2x0 C. to remove hydroxyacetone, inter alia. The loaded extractant is subsequently treated with a base in a separate reactor to convert hydroxyacetone into condensation products. The extractant which has been treated in this way is returned to the extraction stage where the condensation products go into the organic phase and are then separated off in the work-up of the phenol- and acetone-containing organic phase. The examples show that, despite the very complicated apparatus employed for purification by extraction and subsequent reaction of the extracted hydroxyacetone, the organic product phase which is passed to further work-up for the isolation of phenol still contains 500-800 ppm of hydroxyacetone.
It is therefore an object of the invention to provide a process for preparing phenols in which a reduction in the amount of undesirable impurities, in particular hydroxyacetone, prior to the work-up of the product can be achieved in a simple manner.
This object has been achieved by a process for preparing phenols, in which the pH of the reaction product from the acid-catalyzed cleavage of alkylaryl hydroperoxides is set to a value of at least 8 at a temperature of at least 100xc2x0 C. prior to the work-up of the product.
It has surprisingly been found that this simple procedure leads to a drastically reduced content of undesirable by-products, in particular hydroxyacetone, without the complicated apparatus described in the prior art cited being necessary.